/* <div id="back-top"><a href="#top"><span></span></a></div> */
#back-top {
  position: fixed;
  bottom: 5px;
  right: 5px;
}
#back-top span {
  width: 90px;
  height: 90px;
  display: block;
  background: url(back-top.png) no-repeat center;
}